States of Matter

What you'll learn

States of Matter forms a foundational topic in CIE IGCSE Chemistry, tested in both Core and Extended papers. You must understand the three states of matter at a particle level, explain their properties using kinetic particle theory, and describe the physical changes between states. Examiners frequently test your ability to interpret state change diagrams, explain diffusion, and apply particle models to real-world phenomena.

Key terms and definitions

Solid — a state of matter with a fixed shape and fixed volume where particles are arranged in a regular pattern and vibrate about fixed positions.

Liquid — a state of matter with no fixed shape but fixed volume where particles are close together but can move past each other.

Gas — a state of matter with no fixed shape or volume where particles are far apart and move rapidly in random directions.

Diffusion — the net movement of particles from a region of higher concentration to a region of lower concentration as a result of their random motion.

Melting point — the fixed temperature at which a solid changes to a liquid.

Boiling point — the fixed temperature at which a liquid changes to a gas throughout the entire liquid.

Kinetic particle theory — the model that describes how all matter is made of tiny particles that are constantly moving, with the amount of energy they have determining the state of matter.

Sublimation — the direct change from solid to gas without passing through the liquid state.

Core concepts

The three states of matter and particle arrangement

The arrangement, movement and energy of particles determine the properties of each state:

Solids:

• Particles are very close together in a regular, fixed arrangement
• Strong forces of attraction hold particles in fixed positions
• Particles can only vibrate about their fixed positions
• Have the lowest kinetic energy of the three states
• Fixed shape and fixed volume
• Cannot be compressed
• Cannot flow
• High density compared to gases

Liquids:

• Particles are close together but arranged randomly
• Moderate forces of attraction between particles
• Particles can move past each other and throughout the liquid
• Have more kinetic energy than solids
• No fixed shape (take shape of container) but fixed volume
• Cannot be compressed significantly
• Can flow
• High density compared to gases

Gases:

• Particles are very far apart with no regular arrangement
• Very weak forces of attraction between particles
• Particles move rapidly in all directions
• Have the highest kinetic energy of the three states
• No fixed shape or volume (fill container completely)
• Can be compressed easily
• Can flow
• Low density

Changes of state and energy

State changes are physical changes — no new substances are formed and the change is reversible. During a state change, temperature remains constant while energy is transferred.

The six state changes:

1. Melting — solid to liquid (energy absorbed to overcome forces between particles)
2. Freezing — liquid to solid (energy released as forces form between particles)
3. Boiling/evaporation — liquid to gas (energy absorbed to overcome forces)
4. Condensation — gas to liquid (energy released)
5. Sublimation — solid to gas (energy absorbed)
6. Deposition — gas to solid (energy released)

Heating and cooling curves:

When you heat a pure substance and plot temperature against time:

• Sloping sections show temperature rising as kinetic energy increases
• Flat (horizontal) sections show state changes occurring at constant temperature
• During state changes, energy is used to overcome forces between particles rather than increase temperature
• The flat section at melting point is shorter than at boiling point because less energy is needed to partially separate particles (melting) than to completely separate them (boiling)

Evaporation versus boiling:

Evaporation:

• Occurs at the surface of a liquid only
• Happens at temperatures below the boiling point
• Only the particles with sufficient energy escape from the surface
• Rate increases with temperature, surface area, air movement and decreased humidity

Boiling:

• Occurs throughout the entire liquid
• Happens at a specific temperature (boiling point)
• Bubbles of gas form within the liquid

Kinetic particle theory and evidence

The kinetic particle theory states that all matter consists of tiny particles in constant motion. Temperature is a measure of the average kinetic energy of particles.

Key principles:

• Particles in gases move randomly and rapidly
• Collisions between particles and container walls cause gas pressure
• Increasing temperature increases particle speed and kinetic energy
• Forces between particles vary in strength: strongest in solids, weakest in gases

Experimental evidence for particle theory:

Brownian motion:

• Observed when smoke particles in air are viewed under a microscope with a light source from the side
• Smoke particles appear as bright specks moving in random, jerky paths
• This occurs because invisible air particles collide randomly with the visible smoke particles
• Provides direct evidence that air particles are moving randomly and colliding
• At higher temperatures, movement becomes more vigorous as air particles move faster

Diffusion in gases and liquids

Diffusion occurs because particles move randomly from high to low concentration until evenly distributed.

Factors affecting diffusion rate:

• Temperature — higher temperature means particles have more kinetic energy and move faster, so diffusion is faster
• Relative molecular mass (Mr) — particles with lower mass move faster at the same temperature, so diffuse faster
• State of matter — diffusion is faster in gases than liquids because particles in gases move faster and are further apart with fewer collisions

Classic diffusion experiments in CIE IGCSE:

Ammonia and hydrogen chloride demonstration:

• Cotton wool soaked in concentrated ammonia solution is placed at one end of a long glass tube
• Cotton wool soaked in concentrated hydrochloric acid is placed at the other end
• Ammonia gas (NH₃, Mr = 17) and hydrogen chloride gas (HCl, Mr = 36.5) diffuse along the tube
• They meet and react to form a white ring of ammonium chloride: NH₃(g) + HCl(g) → NH₄Cl(s)
• The white ring forms closer to the HCl end because ammonia particles are lighter and diffuse faster
• Demonstrates that rate of diffusion depends on molecular mass

Bromine gas diffusion:

• A brown bromine gas is placed in a gas jar
• An empty gas jar is inverted above it with a glass plate separating them
• When the plate is removed, the brown color gradually spreads upward into the upper jar
• This occurs by diffusion as bromine particles move randomly to fill both jars
• Eventually both jars become an even light brown color
• Takes time because gas particles, though moving fast, collide frequently with air particles

Potassium manganate(VII) in water:

• A single crystal of purple potassium manganate(VII) is placed in water
• The purple color gradually spreads throughout the water without stirring
• Demonstrates diffusion in liquids
• Much slower than gas diffusion because liquid particles are closer together and move more slowly

Factors affecting melting and boiling points

Pure substances have sharp melting and boiling points — a precise temperature at which the state change occurs.

Strength of forces between particles determines melting and boiling points:

• Strong forces = high melting/boiling points (more energy needed to overcome forces)
• Weak forces = low melting/boiling points (less energy needed)

Substances with giant structures (ionic compounds, metals, giant covalent) have strong forces between particles, resulting in high melting and boiling points.

Substances with simple molecular structures have weak forces between molecules, resulting in low melting and boiling points. Many are liquids or gases at room temperature.

Effect of impurities:

• Impurities lower the melting point and broaden the melting range
• Impurities raise the boiling point
• This is why salt is used on roads in winter (lowers freezing point of water)

Worked examples

Example 1: Heating curve interpretation

A pure substance is heated and its temperature recorded every 30 seconds. The graph shows temperature plateaus at 80°C for 3 minutes and at 180°C for 5 minutes.

Question: (a) Identify the melting point and boiling point of this substance. [2] (b) Explain why the temperature does not change during melting even though heating continues. [2] (c) Explain why the plateau at the boiling point is longer than at the melting point. [2]

Mark scheme answers:

(a) Melting point = 80°C ✓ Boiling point = 180°C ✓

(b) Energy is being used to overcome/break forces between particles ✓ (not to increase kinetic energy/temperature) ✓ Alternative: Energy breaks bonds/forces (1 mark only unless second point made)

(c) More energy is needed to separate particles completely (in boiling) ✓ compared to partially separating them (in melting) ✓ Alternative: Particles must overcome stronger forces/move further apart during boiling ✓ so takes longer/needs more energy ✓

Example 2: Diffusion and relative molecular mass

Two gases, X and Y, are released simultaneously from opposite ends of a 100 cm tube. Gas X has Mr = 44 and Gas Y has Mr = 16. They meet and form a white solid.

Question: (a) Explain what is meant by diffusion. [2] (b) Calculate how far from the end where Gas X was released the two gases will meet. [2] (c) State and explain the effect of increasing the temperature on the position where the gases meet. [2]

Mark scheme answers:

(a) (Net) movement of particles ✓ from high to low concentration / due to random motion ✓

(b) Gas Y moves faster because it has lower Mr/is lighter ✓ They meet closer to where Gas X was released / approximately 40 cm from Gas X end ✓ (Exact calculation: rate ratio = √(44/16) = √2.75, so Y travels ~62 cm and X travels ~38 cm)

(c) No effect on position / still meet at same place ✓ Both gases move faster at higher temperature / both affected equally ✓

Example 3: State changes and particle models

Question: Explain the following observations using ideas about particles: (a) A liquid can flow but a solid cannot. [2] (b) A gas can be compressed but a liquid cannot. [2]

Mark scheme answers:

(a) In liquids, particles can move past each other ✓ In solids, particles are in fixed positions/can only vibrate ✓

(b) In gases, particles are far apart/have large spaces between them ✓ In liquids, particles are (already) close together/touching ✓

Common mistakes and how to avoid them

• Mistake: Confusing melting and dissolving. Students write "sugar melts in water" or "salt melts when heated in water." Correction: Melting is a physical change from solid to liquid of a pure substance at its melting point. Dissolving is a substance (solute) spreading throughout a liquid (solvent) to form a solution. Sugar dissolves in water; it melts at 186°C when heated on its own.

• Mistake: Writing that particles "get bigger" when heated or "shrink" when cooled. Correction: Particles themselves do not change size. When heated, particles gain kinetic energy and move faster, moving further apart (in liquids and gases). The substance expands because particles occupy more space, not because particles are bigger.

• Mistake: Stating that temperature increases during a state change. Correction: Temperature remains constant during a state change (at the melting or boiling point). Energy supplied is used to overcome forces between particles, not to increase kinetic energy or temperature.

• Mistake: Confusing evaporation and boiling or using the terms interchangeably. Correction: Evaporation occurs at the surface only, at temperatures below boiling point, and involves only the most energetic particles escaping. Boiling occurs throughout the liquid at a specific temperature (the boiling point) with bubbles forming within the liquid.

• Mistake: Drawing particles in gases as regularly arranged or touching. Correction: In gases, particles must be drawn far apart (relative to particle size) with random, irregular spacing. Particles are not touching and have no pattern.

• Mistake: Confusing properties of states. For example, writing that gases have fixed volume or that liquids can be compressed. Correction: Create a comparison table of properties for the three states and memorize which properties apply to which state. Gases have no fixed shape or volume and can be compressed. Liquids have fixed volume but no fixed shape and cannot be compressed.

Exam technique for States of Matter

• "Describe" questions on particle arrangement: Use three distinct points: arrangement (regular/random, close/far apart), movement (vibrate/move past each other/move rapidly in all directions), and forces (strong/moderate/weak). For full marks you typically need all three aspects.

• "Explain" questions on state changes: Always reference both energy and forces between particles. State that energy is absorbed or released, specify it is used to overcome/break or form forces between particles, and note that temperature remains constant during the change. A complete answer needs all three elements for full marks.

• Diffusion questions: When comparing diffusion rates, always relate your answer to relative molecular mass or Mr values given in the question. State which gas is lighter/has lower Mr, that it moves faster, and therefore diffuses faster. Quantitative questions may require calculating the square root of the Mr ratio.

• Particle diagram questions: When drawing or interpreting particle diagrams, examiners mark strictly on spacing, arrangement and number of particles. In gases, particles must be far apart (at least 5-10 particle diameters). In solids, particles must touch and show a regular pattern. In liquids, particles touch but are randomly arranged.

Quick revision summary

Solids have fixed shape and volume with particles in fixed positions vibrating about fixed points. Liquids have fixed volume but no fixed shape with particles close together able to move past each other. Gases have no fixed shape or volume with particles far apart moving rapidly. State changes occur at constant temperature while energy overcomes forces between particles. Melting, boiling and sublimation absorb energy; freezing and condensation release energy. Diffusion is net movement of particles from high to low concentration, faster at higher temperatures and for particles with lower relative molecular mass. Brownian motion provides evidence for kinetic particle theory.